AGVs having automated guidance systems and thereby capable of operating without a human operator are increasingly common in industrial facilities. AGVs are used for a variety of tasks and functions, of which the most common function is to transport material along predetermined routes. To ensure precise and accurate guidance, AGVs use many guidance methods, including dead reckoning, electrified guide wires, optical systems, inertial guidance systems, magnetic markers, as well as a variety of other systems. Each of these systems has a variety of drawbacks, typically related to system cost and complexity. The largest components of the system costs are the initial installation costs and the cost of each AGV. Issues related to the complexity of the system typically include limited guide path revision flexibility, high costs associated with any guide path revisions, complex and heavy processor use during operation, and for some systems, limited operational accuracy.
In an electrified guide wire system, a conductive wire is buried in the floor of a facility and produces a strong magnetic field. More specifically, the AGV's guidance system senses and tracks an active magnetic field generated by current passing through the buried wire. These active magnetic fields are very easy to accurately and precisely track during operation and therefore have minimal AGV costs in comparison to other types of guidance systems that require more complex sensors and control units on the AGV. However, the initial installation costs of these electrified guide wire systems within the facility in which the AGVs operate are extremely high in comparison to other guidance systems and an electrified guide wire system has extremely limited guide path revision flexibility and high costs associated with any such revisions to the guide paths. More specifically, any change in a route the AGV follows requires tearing up the floor of the facility, removing existing electrified guide path wires and replacing them with rerouted guide path wires, all resulting in significant inconvenience or down time for the facility. As manufacturing and distribution facilities are increasingly implementing flexible techniques to allow switching between products being produced or distributed, the AGVs in these facilities are commonly using optical or inertial guidance systems, which do not require extensive facility renovations every time a minor change is made to the guide path of the AVGs.
Optical guidance systems have made significant strides in improved accuracy, but typically require expensive sensors and controllers with substantial processing power to process the graphical images used in guidance of the AGV, which increases the cost and complexity of each AGV. Inertial guidance systems, similar to optical guidance systems, have significantly improved their capabilities, however, inertial guidance systems using encoders and gyroscopes also typically require significant processing power. As such, the guidance systems for these AGVs are generally expensive and complex in comparison to the guidance systems for AGVs that follow an electrified guide wire. While optical and inertial guidance systems do not require tearing up the floor of a facility to make route changes, similar to most guidance systems, any route changes may require substantial time and costs for the system to learn the new route. In optical and inertial guidance systems, at least one AGV typically must be taught the new route and in some systems, a new map of the facility must be created, which can be time consuming and require specialized expertise. As such, the components on the AGV relating to the guidance system may have substantially greater costs and complexity than similar components on any AGV in an electrified guide wire system. The costs associated with the AGVs learning new routes are much higher for inertial and optical guidance systems than electrified guide wire systems, when the costs relating to facility renovations related to new routes are excluded. In comparison to AGVs that follow electrified guide wires, the advantage of AGVs with inertial and optical guidance systems is that they allow high guide path flexibility, and route changes require little facility renovations. For example, at most, some optical guidance systems only require new markers, targets or reflectors to be added to the facility in relation to any guide path changes.
As the use of AGVs in the material handling industry has increased, there has been a corresponding growth and desire for lower cost AGVs that include flexibility regarding route revisions within a facility, similar to inertial and optical guidance systems, with the lower sensor costs and ease of learning new paths for AGVs used in following electrified guide wire systems. In an attempt to meet these desires, some manufacturers are using AGVs that follow a passive magnetic pathway, which instead of using an expensive buried electrified guide wire, uses passive magnetic materials typically applied to the floor of the facility. Examples of such passive magnetic materials include magnetic tapes, paints, bars, markers and other magnetic materials. Due to the weaker magnetic field generated by these passive magnetic materials, some AGVs have experienced issues in accurately and precisely tracking the guide paths formed by the passive magnetic materials. While the installation costs are much lower and route revisions are much cheaper than electrified guide wire systems, the weaker magnetic fields of these passive magnetic materials may limit the ability of traditional magnetic sensors on the AGV, previously used with electrified guide wires, to determine the precise location of the weaker magnetic signal, especially in facilities having strong background or ambient magnetic fields.
Two types of passive magnetic markers are commonly used, continuous magnetic pathways or discrete magnetic markers. One problem with discrete magnetic markers is that the AGV must be presumed to be in alignment with the path defined by the discrete magnet markers. Therefore, the AGV must first be manually aligned very carefully with the pathway of magnet markers. During operation, if errors compound, such as a heading error increasing to a point where correction is not possible, the sensors on the AGV may be too far displaced from the next marker to obtain a valid reading of the next marker. Missing or displaced magnetic markers are also problematic, as once the AGV is misaligned with the actual path, most current methods of using discrete markers are unable to correct the AGV's path sufficiently to detect the next correctly located discrete marker in the pathway. More specifically, even when the AGV is traveling along a straight pathway, if the next expected magnetic marker is displaced to one side from its proper position, the AGV may initiate a turn based on the location of the displaced marker, which may cause the heading of the AGV to be too far deviated from the correct pathway to locate the subsequent properly placed marker. These issues are particularly acute in areas where an AGV must make a turn.
Methods of using discrete magnet markers typically require a variety of external apparatuses and complex methods to follow changes in the path such as curves and junctions or divergences in the path. To adjust and correct for potential issues, many methods have been proposed that encode information about path changes in the magnetic markers. These encoding systems are typically limited to a small amount of information and may even require many individual magnets to form a single marker to encode enough information. The requirements for these individual, specialized magnets increases the cost of installation and overall complexity of the system, while the practicality of the system is limited in view of the limited amount of information capable of being encoded.
While some systems have attempted to replace the discrete magnetic markers in turns with a continuous marker, such as a magnetic bar, these bars are typically expensive to install, may require facility renovations and may require expensive custom radius magnetic bars in the turns. To reduce the initial installation cost, eliminate the problems described above with discrete markers and provide increased flexibility relating to changes in the routes of the AGVs within a facility, some systems are using continuous magnetic markers such as magnetic tape, magnetic paint, and adhesive materials having magnetic properties or any combination thereof. However, due to the typically weaker magnetic fields emitted by these continuous markers, as compared to electrified guide wires and even discrete magnetic markers, the magnetic sensors used on these AGVs have increased in cost and the processing power required for tracking has also increased to ensure accurate and precise tracking similar to the tracking obtained in electrified guide wire systems with a stronger magnetic field. For example, many current systems require the use of twelve different sensors in an array, such that the magnetic field strength from each sensor may be used to determine which sensors are over the path marker. Based on the determination of which of the twelve sensors are over the marker, the AGV's processing unit then determines the offset of the vehicle from the centerline of the magnetic marker path and adjusts the travel path of the AGV. The cost of these systems is increased due to the number of sensors used, and the amount of processing power required to process the twelve different signals. The AGV's cost is also increased in that typically at least two sensors are also used to detect stray, ambient, or background magnetic fields. The effectiveness of these background magnetic sensors is limited since these sensors only detect one component of the three components in the magnetic field.
Another problem with current magnetic sensor assemblies following a passive and continuous magnetic marker is that the algebraic relationship used to calculate the location of the AGV is not only sensitive to the width of the marker, but also the height of the sensor from the magnetic marker. As such, minor deviations in the width of the marker or height of the sensor on the AGV may cause variations in locating a magnetic marker. As the height of the sensor from the magnetic marker is required in calculating the position of the magnetic marker, it is very important that the height of the sensor assembly during the assembly of the AGV precisely matches the specified height, raising costs of assembly or requiring specific calibration of each AGV in relation to height. The requirement for the height to be precisely known is also problematic as it may vary depending on the amount of load the AGV is carrying or due to the variations in the floor of the facility. Additionally, the requirement for the width of the magnetic marker to be consistent over its entire length is problematic. For example, if magnetic paint is used, it is possible for variations in width to occur over the miles of routes typically found in a facility, especially over time in view of the wear caused by the AGVs and other vehicles and people continually passing over the magnetic marker.
Similar to the above described system and method, other magnetic systems and methods have been developed wherein at least two sensors are placed a known distance apart on the AGV and angled such that their sense axes meet at approximately the location of the centerline of the magnetic tape, if the AGV is centered along the magnetic marker. This method is also subjective to ambient magnetic fields, the width of the magnetic tape as well as the height of the vehicle, similar to a sensor directly placed over the center of the marker. Very careful alignment and calibration is also required to ensure precise and accurate guidance by the AGV as well as high precision during manufacturing regarding the height of the sensor assembly, the distance apart of the two magnetic sensors, and relative angles of the sensors.